Multimode fibers have constantly evolved from the very beginning of optical communications industry through the recent and on-going explosion of the Ethernet traffic. Enabled by VCSEL technology, high-speed multimode optical fibers, such as OM4 fibers (which are laser-optimized, high bandwidth 50 μm multimode fibers, standardized by the International Standardization Organization in document ISO/IEC 11801, as well as in TIA/EIA 492AAAD standard), have proved to be the medium of choice for high data rate communications, delivering reliable and cost-effective 10 to 100 Gbps solutions. The combination of Wide-Band (WB) multimode fibers with longer-wavelengths VCSELs for Coarse Wavelength Division Multiplexing (CWDM) is an interesting option to be considered in order to meet the future increase of demand.
However, the high modal bandwidth of OM4 fibers has until now only been achieved over a narrow wavelength range (typically 850 nm+/−10 nm). The feasibility of Wide-Band (WB) multimode fibers satisfying OM4 performance requirements over a broader wavelength range is a challenge to overcome for next generation multimode systems.
The OM4 fiber performance is usually defined by an Effective Modal Bandwidth (EMB) assessment at a given single wavelength. For instance, OM4 fibers should exhibit EMB larger than 4,700 MHz-km at a wavelength of 850 nm. The achievement of such high EMB values requires an extremely accurate control of refractive index profile of multimode fibers. Up to now, traditional manufacturing process cannot guarantee so high EMB, and generally it is hard to accurately predict the EMB values from refractive index profile measurements on core rod or cane, especially when high EMB (typically larger than 2,000 MHz-km) are expected, meaning the fibre refractive index profile is close to the optimal profile. As a matter of fact, EMB are directly assessed on fibers.
In order to minimize modal dispersion, the OM4 fibers generally comprise a core showing a refractive index that decreases progressively going from the center of the fiber to its junction with a cladding. In general, the index profile is given by a relationship known as the “α profile”, as follows:
            n      ⁡              (        r        )              =                            n          0                ⁢                              1            -                          2              ⁢                                                Δ                  (                                      r                    a                                    )                                α                                                    ⁢                                  ⁢        for        ⁢                                  ⁢        r            ≤      a        ,where:                n0 is a refractive index on an optical axis of a fiber;        r is a distance from said optical axis;        a is a radius of the core of said fiber;        Δ is a non-dimensional parameter, indicative of an index difference between the core and a cladding of the fiber; and        α is a non-dimensional parameter, indicative of the general shape of the index profile.        
The Alpha parameter (a) that governs the shape of this graded-index core can be tuned to maximize the modal bandwidth at 850 nm of OM4 multimode fiber, the typical operating wavelength of high-speed data communications. A given alpha parameter value is generally selected to offer an optimum EMB as illustrated in document “WideBand OM4 Multi-Mode Fiber for Next-Generation 400 Gbps Data Communications” by Molin et al. ECOC 2014.
The Effective Modal Bandwidth (EMB) is assessed by a measurement of the delay due to the modal dispersion, known under the acronym DMD for “Dispersion Modal Delay” graphical representation. It consists in recording pulse responses of the multimode fiber for single-mode launches that radially scan the core. It provides a DMD plot that is then post-processed in order to assess the minimal EMB a fiber can deliver at a given wavelength. The DMD measurement procedure has been the subject of standardization (IEC 60793-1-49 and FOTP-220) and is also specified in Telecommunications Industry Association Document no. TIA-455-220-A. Each DMD metric, or DMD value, is expressed in units of picoseconds per meter (ps/m). It determines the delay between the fastest and the slowest pulses traversing the fibre considering a collection of offset launches normalized by fiber length. It basically assesses a modal dispersion. Low DMD value, i.e. low modal dispersion as measured by DMD generally results in higher EMB.
Basically, a DMD graphical representation is obtained by injecting a light pulse having a given wavelength at the center of the fiber and by measuring the pulse delay after a given fiber length L, the introduction of the light pulse of a given wavelength being radially offset to cover the entire core of the multimode fiber. Individual measurements are thus repeated at different radial offset values so as to provide cartography of the modal dispersion of the examined multimode fiber. The results of these DMD measurements are then post-processed to determine an effective transfer function of the optical fiber, from which a value of EMB may be determined.
Nowadays, all multimode fibre manufacturers perform DMD measurements and EMB assessment at a single wavelength only of their whole production: typically at 850 nm+/−2 nm for OM4 qualification and at 850 nm+/−10 nm for OM3 qualification.
With the advent of new multimode fibre application, requiring high EMB over a wide operating window, one of the main concerns of the multimode fibre manufacturers is to have the ability to easily assess the EMB over a wide wavelength range, for example between 850 nm and 950 nm.
Using the aforesaid classical measurement procedure (comprising a series of DMD measurements and an EMB assessment at a single wavelength) to assess the optical fiber's EMB over a range of wavelengths, i.e. at a plurality of wavelengths, would require performing several measurement procedures at said wavelengths adequately spread over the wavelength range of interest. However, making distinct independent DMD measurements to qualify the optical fiber's EMB at multiple wavelengths greatly leads to increase measurement time and the cost of measuring and producing wide-band multimode fibers. Such a solution would notably require implementation of several light sources each emitting in a distinct wavelength and several corresponding detectors, which would represent a complex and costly operation.
Therefore, there remains a need for a simple and low-cost method for identifying during production multimode fibers that guarantee high modal bandwidth over a wide wavelength spectrum only from a single wavelength characterization.
The U.S. Pat. No. 8,351,027 proposes to use a metric derivable from DMD measurement in combination with industry-standard metrics such as Effective Modal Bandwidth and DMD to obtain a more accurate prediction of multimode fibre channel link performance as measured by BER testing. The metric can be used to select or verify fiber performance at a wavelength close to the wavelength of the DMD measurements.
The invention provides in at least one embodiment a method that enables to guarantee the EMB of a wide-band multimode fiber over a relatively large spectral window while characterization is restricted to a single wavelength.
In another at least one embodiment the invention provides a method for selecting wide-band multimode fibers from a batch of multimode fibers that is simple to implement and that reduces multimode fiber measurement costs.